Epoxy resins have excellent mechanical properties and thermal properties, and therefore are widely used in various fields. Imidazole is used as a curing agent for curing such epoxy resins. But, a problem of an epoxy resin-imidazole mixed liquid is that it cannot be used as a one-component type because curing starts early and it is thickened or gelled in long-term storage.
Accordingly, the use of an imidazole acid addition salt in which hydroxybenzoic acid is added to imidazole, as the curing agent (see Patent Document 1), and the use of an inclusion compound of a tetrakisphenol compound (for example, 1,1,2,2-tetrakis(4-hydroxyphenyl)ethane (hereinafter referred to as TEP)) and imidazole as the curing agent (see Patent Documents 2 and 3) are proposed. In addition, the present inventors propose a cured resin composition using an inclusion compound of an isophthalic acid compound and imidazole (see Patent Document 4). But, although these achieve a certain effect, these are not satisfactory yet.
Epoxy resin compositions containing an epoxy resin, a curing agent, a cure accelerator, and other additives are used as encapsulation materials for semiconductor devices, such as transistors, ICs, and LSIs, and electrical components. For the purpose of improving the preservation stability of the epoxy resin compositions, the use of an inclusion compound comprising an imidazole compound or an amine compound as a guest compound and TEP as a host, as the cure accelerator, is proposed (see Patent Document 5). The inclusion of an imidazole compound or an amine compound can promote an improvement in the preservation stability of the encapsulation material at ordinary temperature, compared with a case where these compounds are used alone or in combination. But, it is not sufficiently satisfactory for an encapsulation material composition that addresses the fine specifications of semiconductors which have been significantly advanced in recent years.
In addition, salts of aliphatic divalent carboxylic acids and imidazole compounds are known (see Patent Documents 6 to 10), but all have a content ratio of 1:2.